1. Field of the Invention
The invention relates to a method for producing a saccharide mainly including glucose, by glycosylating a plant fiber material and separating the obtained saccharide.
2. Description of the Related Art
It has been suggested to produce a saccharide mainly including glucose or xylose, from a plant material, which is a biomass, such as cellulose or hemicellulose by degrading squeezed sugarcane residues (bagasse) or wood chips and effectively use the produced saccharide as food or fuel, and this process has been put into practice. In particular, a technology by which a monosaccharide obtained by degrading plant fibers is fermented to produce an alcohol such as ethanol as fuel has attracted attention. A variety of methods have been heretofore suggested for producing a saccharide such as glucose by degrading cellulose or hemicellulose (for example, Japanese Patent Application Publication No. 8-299000 (JP-A-8-299000), Japanese Patent Application Publication No. 2006-149343 (JP-A-2006-149343), Japanese Patent Application Publication No. 2006-129735 (JP-A-2006-129735), and Japanese Patent Application Publication No. 2002-59118 (JP-A-2002-59118)), and a typical method includes hydrolyzing cellulose by using sulfuric acid such as dilute sulfuric acid or concentrated sulfuric acid or hydrochloric acid (JP-A-8-299000). A method in which cellulase enzyme is used (JP-A-2006-149343), a method in which a solid catalyst such as activated carbon or zeolite is used (JP-A-2006-129735), and a method in which pressurized hot water is used (JP-A-2002-59118) are also available.
However, a problem associated with the method by which cellulose is degraded by using an acid such as sulfuric acid is that the acid serving as a catalyst and the produced saccharide are difficult to separate from the hydrolysis reaction mixture obtained by hydrolysis. This is because glucose that is the main component of the cellulose hydrolysis product and the acid that serves as a hydrolysis catalyst are both soluble in water. Removal of the acid by neutralization or ion exchange from the hydrolysis reaction mixture is not only troublesome and costly, but it is also difficult to remove the acid completely and the acid often remains in the process of fermentation for ethanol. As a result, even when pH is optimized from the standpoint of activity of yeast in the process of fermentation for ethanol, concentration of salt increases, thereby reducing the yeast activity and decreasing the fermentation efficiency.
In particular, when concentrated sulfuric acid is used, the sulfuric acid is very difficult to remove to the extent such that yeast is not deactivated in the process of fermentation for ethanol and such a removal requires significant energy. By contrast, when dilute sulfuric acid is used, the sulfuric acid is relatively easy to remove. However, it is necessary to degrade cellulose under high temperature conditions, which is energy consuming. In addition the acid such as sulfuric acid and hydrochloric acid is very difficult to separate, collect, and reuse. Thus, the use of these acids as a catalyst for producing glucose is a cause of increased cost of bio-ethanol.
With the method in which pressurized hot water is used, it is difficult to adjust the conditions, and it is difficult to produce glucose with a stable yield. In addition, in this method, even glucose is degraded, thereby reducing the yield of glucose. Moreover, the activity of yeast is reduced by degraded components and fermentation may be inhibited. Another problem is associated with cost because the reactor (supercritical processing apparatus) is expensive and has poor durability.